Photovoltaic (PV) or solar cells are material junction devices which convert radiation into direct current (DC) electrical power. When exposed to sunlight (consisting of energy from photons), the electric field of solar cell p-n junctions separates pairs of free electrons and holes, thus generating a photo-voltage. A circuit from n-side to p-side allows the flow of electrons when the solar cell is connected to an electrical load, while the area and other parameters of the PV cell junction device determine the available current. Electrical power is the product of the voltage times the current generated as the electrons and holes are separated by a current collecting junction.
A cross-sectional diagram of a known semiconductor based solar cell 100 is shown in FIG. 1. The solar cell has a top grid 101 that serves as the top electrode, an anti-reflection layer 102, an n-type layer 103, a p-type bulk 104, point contacts 105 connected to a back metal layer 107 that serves as the back electrode and a dielectric layer 106.
Power is generated by an external current between top electrode and back electrode which is sustained by charge carriers being released from the cell by radiation. In order to obtain high efficiency with silicon solar cells, it is necessary to reduce carrier recombination at the front and back surfaces. This is usually done with a process called passivation. When the solar cell absorbs a photon an electron-hole pair is created. If these carriers recombine before they are collected at a semiconducting junction, they are lost and cannot contribute to the cell's current. Because surfaces present a discontinuity with a high density of dangling bonds, they are primary sites for recombination. The problem is made worse in advanced cell designs that have long lifetime substrates where there is a high probability that carriers will reach a surface before they reach a junction.
An attractive method to passivate the surface is to deposit an insulator layer also called a passivation layer on the body of the cell. Most commonly, Si3N4 or SiO2 are used as passivation layer material. SiO2 has as an attractive property that it forms an interface with a minimum number of dangling bonds. However, it has a low index of refraction that is unsuitable to form an anti-reflection coating. Si3N4 does not form as good an interface with a low number of dangling bonds. However, it has an index of refraction near the ideal value for use with silicon.
With either material, it is possible to add charge to a passivation layer. Nitride forms with a net positive charge using CVD or PVD. Silicon dioxide also forms with a net positive fixed charge. On p-type material, the positive charge inverts the surface, creating a layer of electrons (minority carriers) and an absence of holes (majority carriers). The traps will charge with electrons, but there are no holes to discharge them, thereby neutralizing them.
In certain cell designs, such as the PERL or PERC cells, or laser fired back contact cells, it is desirable to impart a negative charge to the passivation layer. These are point contact cells, as for instance shown in FIG. 1, which have back contacts in the form of small localized regions, with the remaining back area covered with a passivation layer. These contacts form a CMOS structure that will drain off an inversion layer. Because of the long lifetime in the substrate, the minority carrier generation rate is too low, and the inversion layer cannot form. An accumulation layer is preferred because it contains readily available majority carriers, and can therefore respond much more rapidly. A negative charge is required to form an accumulation layer in p-type material.
Negative charge, however, is not compatible with a traditional solar cell process. Such a process uses hydrogen to passivate defects in the bulk and at the interface between the passivation layer and the semiconductor. Hydrogen has a positive charge, and will neutralize negative charge within the passivation layer. In many cases, the passivation layer is formed with an excess of hydrogen to act as a source for hydrogen passivation. This further complicates realizing a net negative charge in the passivation layer.
Therefore, there is a need for a negative charged passivation layer. It would also be desirable to provide such a negatively charged layer while still maintaining the availability of hydrogen for passivation and for a process to form such a negative charged passivation layer.